No technology has had as profound an influence as the integrated circuit. Most of the wonders of our society would be impossible without it. A single example, the modern computer, plays a role that affects the wellbeing of people in every walk of life and in every country.
It has been confidently predicted for many years that future generation integrated circuits would include or even take the fundamental form of integrated photonic rather than electronic devices. While many uses may involve optical pumping, most significant impact is expected to depend upon electrical pumping. From this standpoint, that is from the standpoint of laser operation, per se, and also from the standpoint of functions to be performed, most valuable implementation will take the form of the Opto Electronic Integrated Circuit. Included electronics--generally semiconductor electronics--will serve to operate lasers as well as to interact between photonics and electronics generally.
Contemplated uses for integrated lasers--largely represented by OEIC's--include integration with the driving electronics itself as well as inclusion of elements to provide for interaction both within photonic parts of the circuit and between photonic and electronic parts. Examples of purely photonic interaction include laser-to-detector. There are many examples of the second category.
While overcoming limitations of present day electronic computers will, by all estimates, lead to assignment of at least some computer functions to lasers, intermediate development is expected to take the form of optical interconnection to overcome present limits interposed by hard wiring. Realization of optical interconnect, chip-to-chip as well as within chip, will be dependent upon a high degree of integration consistent with low cost.
Other uses for OEIC's as identified in recent literature include imaging, image processing and optical gas detection.
Full implementation of the integrated laser, e.g. of the OEIC, will take advantage of very profound emerging processing advances. Demonstrated techniques have, for example, depended on close lattice matching, but have also taken advantage of defect minimization afforded through growth of extremely thin layers in non-lattice matched epitaxy as well. Such considerations have been quite valuable in conjunction with evolving Quantum Well structures with their well appreciated performance advantages (low threshold, narrow bandwidth, high brightness, high effiency). See Y. Arakawa and A. Yariv, IEEE J. Quantum Electron. QE-21, 1666 (1985). Remarkable developments confirm the likelihood of commercial abricattion of discrete QW laser structures in the near future. Dislocation densities have been reduced. Useful Distributed Feed Back mirrors have been built without need for cleavage (by Molecular Beam Epitaxy as well as by Metal Organic Chemical Vapor Deposition.
It is confidently expected that most valuable use will entail a high level of integration. The most significant obstacle to laser integration is power dissipation. The present level of silicon Large Scale Integration is attributable to the very low power requirements for operation. In the limit, power dissipation, some finite fraction of power input, limits the number of devices that may be integrated on a chip, or, in more general terms, the fraction of wafer area devoted to active use. Now available silicon LSI "megachips", soon to yield to still greater miniaturization and integration density, are critically dependent on this consideration. By the same token, relative lack of progress in laser integration must be ascribed to the relatively high required threshold power levels.
Extensive literature clearly shows that workers, worldwide look to QW laser structures as promising from the standpoint of threshold power. Much of the reported work until recently has dealt with edge-emitting structures in which the initially emitted beam is parallel to the major substrate surface. Such structures, while very significant, are not regarded as offering the lower threshold power levels needed for desired integration density.
Very promising recent results involve Quantum Well Vertical Cavity Lasers. As the terminology implies, the emitted beam is now initially non-parallel --generally normal--to the major substrate surface, or more generally, non-parallel to the major dimensions of the quantum well (parallel to the critical quantum well dimension). Representative work is described by Gourley et al in Applied Physics Letters, 49 (9), p. 489 (1 Sept. 1986). In this work conducted at Sandia National Laboratories, the authors describe a passive structure (no non-linear active material) consisting of a cavity of GaAs bounded by DFB mirrors of alternating layers of GaAs and AlAs. The quarter-wave mirror structures in the described experiment consisted of eight layers each of the two materials. Excellent properties observed showed a 54 .ANG. centered transmission band at a wavelength of 0.956 nm. The authors report certain properties from which it is possible to calculate absorption/scattering losses of their structure. Properties reported include a peak transmittance of 45%, as well as their calculated "ideal structure" transmittance of 68%.
Much of the work concerning QWVCL's has involved direct use of the initially vertically directed emitted beam. This is of value, for example, in chip-to-chip communication and in certain other instances in which laser output is to be coupled with external circuitry. The vertical structure, however, also offers low threshold-related advantages for use involving communication within the chip (or under other circumstances in which coupling is in a direction parallel to the major substrate plane). For such purposes, the initially non-parallel beam as emitted by the laser may be redirected, for example, by use of a mirror or prism external to the cavity itself. Such mirrors, likely DFB's, of the same nature discussed in conjunction with the cavity itself could be fabricated as part of an integrated circuit. In passing, it may be noted that while "surface emitting" structures are generally though of in terms of true vertical cavities, there is no reason why they cannot be at some other angle to facilitate desired operation, emitting at 90.degree. or any other out-of-plane angle, or even emitting in-plane. It is implicit, as well, that "surface emitting" may not mean free surface emitting. "Surface emitting" has reference only to the surface of the concerned mirror defining the laser cavity. This "surface" may, in fact, be an interface with some other material/element, e.g., may relate to the interface with a prism or other element designed to direct the beam. (Use of the term, Quantum Well Vertical Cavity Laser, is designed to avoid some of these implied, but unintended limitations).
The Gourley et al article is representative of the most advanced state of the art at least in terms of DFB mirror cavities. It serves as basis for explaining the general view that a small number of quantum wells is inadequate for lasing. A Fabray-Perot etalon has a net maximal transmission of T.sup.2 /(T+A).sup.2, a relationship well-known to workers in relevant fields and found in many textbooks on optics. T is the transmittance of a single mirror of the Fabray-Perot etalon and A is the fraction of energy absorbed by the same mirror. Energy not transmitted or absorbed is reflected. For quarter wavelength interference mirrors in their 8 layer pair GaAs-AlAs mirror structure, one obtains reflectivity very close to 93.6% and a transmittance of 5.2% per mirror. Subtracting the sum of 93.6% +5.2%=98.8% from 100% yields a loss of 1.2% per single pass. Lasing requires a gain of greater than 1.2% in the active structure to offset this loss. Gains offered by quantum well structures are significantly less than 1.2%. For example, the authors of "Ultimate Limit in Low Threshold Quantum Well GaAlAs Semiconductor Lasers", Applied Physics Letters, Vol. 52, pp. 88-90, 11 Jan. 1988, reported experimental results which indicate a maximum achievable gain of 0.3% for a SQW based on quantum wells made of GaAs a well-studied representative member of the promising class of compound semiconductors.
For the author's mirror structure, reflectance values needed for lasing for a small number of quantum wells (four or fewer), twenty or more mirror pairs are required for needed reflectance. Based on the very conservative estimate that the 1.2% loss for the eight pair cavity is not increased at all for the required number of pairs, still results in the conclusion that lasing threshold has not been reached. Stated differently, the results of this reported work lead directly to the conclusion that a minimum of four quantum wells is necessary to offset loss and thereby achieve lasing in the best available materials.
It is the general view that lasing materials within the compound semiconductor families (III-V's, II-VI's and usual ternary, quaternary and higher order materials) have similar maximum gains, i.e. within the range 0.2%-0.4%. All numbers are for single quantum well structures with effective gain for multiple structures increasing linearly as the number of wells increases (2.times.0.2%=0.4% to 4.times.0.2%=0.8% or 0.4%-0.8% for a duplex structure). In addition, cavity efficiency is dependent upon mirror efficiency which, in turn, varies .about.as the reciprocal of the difference in refractive index between the two successive layered mirror materials (.DELTA. n). Since, in general, refractive index for suitable mirror materials varies less for increasing wavelength, .DELTA. n values are typically less so that a greater number of layers is generally required for the necessary cumulative reflectivities.
The work reported above represents the worldwide view that attainable vertical cavity structures (QWVCL's) require a threshold level of at least 3,000 amperes/square centimeter (3mA/100 .mu.m.sup.2) in order to offset unavoidable losses in the cavity. The primary implication of this conclusion is relevant to minimum required power dissipation and, therefore, to maximum integration density. This understanding is equivalent to the accepted view that lasing in QW structures is not attainable for structures containing fewer than a minimum of four quantum wells. Whereas requirements vary at different wavelengths and in accordance with other factors (e.g. available .DELTA. n's), the general view is that such considerations are not at variance--in fact often suggest still greater numbers of wells.
Much of the above is concerned primarily with light pumping. Accordingly, conclusions in the literature relative to required gain, numbers of quantum wells, etc. are largely derived from reported work on light pumping. As indicated, anticipated value, while not ignoring light pumping, is primarily in electrical pumping. All of the reported work is relevant to electrical pumping. On the other hand, it is well known that achievements in optical pumping have not been duplicated in the electrical analog. For example, whereas there is a body of reported work on optically pumped devices operating cw at room temperature, reported electrically pumped devices, have, in the main, been pulsed, or if cw, have been cooled.